Rework PostfixEvaluator to use a UniqueString type

Profiling of stack unwinding found that a large portion of the time was spent
doing string comparisons as a result of using std::map<string,...> in
PostfixEvaluator. This patch introduces a UniqueString type which atomizes
strings so we can simply do pointer comparisons on them, and makes
PostfixEvaluator use it internally.

[Ted Mielczarek]

Neglected to mention, this patch is originally by Julian Seward.